Inhibition of MDM2 by RG7388 confers hypersensitivity to X‐radiation in xenograft models of childhood sarcoma

Background Curative therapy for childhood sarcoma presents challenges when complete resection is not possible. Ionizing radiation (XRT) is used as a standard modality at diagnosis or recurrence for childhood sarcoma; however, local recurrence is still problematic. Most childhood sarcomas are TP53 wild type at diagnosis, although approximately 5–10% have MDM2 amplification or overexpression. Procedures The MDM2 inhibitor, RG7388, was examined alone or in combination with XRT (20Gy given in 2 Gy daily fractions) to immune‐deficient mice bearing Rh18 (embryonal) or a total of 30 Gy in 2 Gy fractions to mice bearing Rh30 (alveolar) rhabdomyosarcoma xenografts. RG7388 was administered by oral gavage using two schedules (daily ×5; schedule 1 or once weekly; schedule 2). TP53‐responsive gene products (p21, PUMA, DDB2, and MIC1) as well as markers of apoptosis were analyzed. Results RG7388 showed no significant single agent antitumor activity. Twenty Grays XRT induced complete regressions (CR) of Rh18 with 100 percent tumor regrowth by week 7, but no tumor regrowth at 20 weeks when combined with RG7388. RG7388 enhanced time to recurrence combined with XRT in Rh30 xenografts compared to 30 Gy XRT alone. RG7388 did not enhance XRT‐induced local skin toxicity. Combination treatments induced TP53 responsive genes more rapidly and to a greater magnitude than single agent treatments. Conclusions RG7388 enhanced the activity of XRT in both rhabdomyosarcoma models without increasing local XRT‐induced skin toxicity. Changes in TP53‐responsive genes were consistent with the synergistic activity of RG7388 and XRT in the Rh18 model. Pediatr Blood Cancer 2015;62:1345–1352. © 2015 Wiley Periodicals, Inc.